A liquid crystal display device is used in various kinds of liquid crystal display units, including a monitor of a notebook-sized personal computer or a desktop personal computer, a viewfinder of a video camera, a projection display and a television. The liquid crystal display device is further utilized as an optoelectronics-related device such as an optical printer head, an optical Fourier transformation device and a light valve. As a liquid crystal display device that has been applied so far, a display device using a nematic liquid crystal is predominantly applied, and a practical application has been made for a liquid crystal display device having a twisted nematic (TN) mode in which a direction of alignment of liquid crystals in the vicinity of one substrate, and a direction of alignment of liquid crystals in the vicinity of the other substrate are twisted at an angle of 90 degrees, a super twisted nematic (STN) mode in which the directions of alignment are ordinarily twisted at an angle of 180 degrees or more, or a so-called thin-film-transistor (TFT) mode in which a thin-film transistor is used.
However, a viewing angle at which an image can be properly visually recognized is narrow in the liquid crystal display devices, and when the image is viewed from an oblique direction, luminance and contrast may be occasionally decreased, and luminance inversion may be occasionally caused in a halftone. The issue of the viewing angle has been recently improved by a liquid crystal display device having a TN mode in which an optical compensation film is used, a multi-domain vertical alignment (MVA) mode in which a technology of vertical alignment and a technology of protrusion structure are simultaneously applied (see Patent literature No. 1), an in-plane switching (IPS) mode according to a transverse electric mode (see Patent literature No. 2), or the like.
A development of technology on the liquid crystal display device has been achieved not only by an improvement of a driving mode and a device structure as described above but also by an improvement of a component used for the display device. Among the components used for the display device, in particular, a liquid crystal alignment film is one of important elements relating to a display quality of the liquid crystal display device, and a role of the liquid crystal alignment film becomes increasingly important with achieving a high quality of the display device year by year.
The liquid crystal alignment film is required to uniformly control alignment of molecules of liquid crystals for developing uniform display characteristics in the liquid crystal display device. Therefore, the liquid crystal alignment film is required to uniformly align liquid crystal molecules on a substrate in one direction to further develop a fixed tilt angle (pretilt angle) from a substrate surface.
Moreover, in order to realize an improvement in contrast and extension of a viewing angle range in the image display unit, as an optical compensation film or a phase difference film, for example, a stretched film having refractive index anisotropy or a film prepared by aligning and polymerizing a polymerizable liquid crystal compound is used.
In general, the liquid crystal alignment film is formed using a liquid crystal aligning agent. The liquid crystal aligning agent that is mainly used is currently in the form of a solution prepared by dissolving polyamic acid or soluble polyimide into an organic solvent. Such a solution is applied to the substrate, and then a film is formed by a means such as heating, and thus a polyimide liquid crystal alignment film is formed. Various kinds of liquid crystal aligning agents other than polyamic acid are also examined, however, are seldom practically utilized in view of heat resistance, chemical resistance (resistance to liquid crystals), applicability, liquid crystal alignment properties, electric characteristics, optical characteristics, display characteristics and so forth.
Industrially, a rubbing method that is simple and allows high-speed treatment in a large area is widely applied as an alignment treatment method. The rubbing method applies treatment for rubbing a surface of the liquid crystal alignment film in one direction by using a fabric prepared by transplanting fibers of nylon, rayon, polyester or the like, and a uniform alignment of the liquid crystal molecules can be obtained by the treatment. However, dust or static electricity generation or the like is caused by the rubbing method. Thus, an alignment defect or an influence of dust or static electricity generation on the liquid crystal device is regarded as a problem. Moreover, in the case of a patterned phase difference film, control of an alignment pattern by the rubbing treatment is difficult.
Consequently, a development has recently been made for a liquid crystal alignment control method in place of the rubbing treatment. With regard to a photoalignment method by which alignment treatment is applied by irradiation with light, many alignment mechanisms have been proposed, such as a photolysis method, a photoisomerization method, a photodimerization method and a photocrosslinking method (see Patent literature No. 3, Patent literature No. 4, Patent literature No. 5 and Patent literature No. 6.). The photoalignment method applies non-contact alignment, which is different from the rubbing method. In principle, a smaller amount of dust or static electricity is generated by the photoalignment method, as compared with the rubbing treatment.
An improvement in performance as the liquid crystal display device can be expected by controlling a state of alignment of molecules in a liquid crystal monomolecular layer in contact with the liquid crystal alignment film by using a liquid crystal alignment film having good alignment properties to which the alignment treatment is applied by the photoalignment method.
A passive glasses 3D display has been recently practically applied as one of 3D display modes. According to the 3D display, a phase difference plate is mounted on a panel of the liquid crystal display. As the phase difference plate, an examination has been made for a patterned phase difference plate prepared by aligning the polymerizable liquid crystal compound to the liquid crystal alignment film to which the alignment treatment is applied by the photoalignment method. Patterning of the phase difference plate is performed by irradiating a film with polarized ultraviolet light having a different polarization direction to prepare the liquid crystal alignment film, and then applying polymerizable liquid crystals to the film to allow patterning alignment. Upon preparing the patterned phase difference plate, time of exposure to polarized ultraviolet light can influence productivity in a process, but the productivity tends to be further increased as the time of exposure thereto is shorter. Accordingly, in order to shorten the time of exposure to polarized ultraviolet light, an improvement in sensitivity of the photoalignment film to light has been required.
Moreover, a plastic such as triacetyl cellulose (TAC) and a cyclic olefinic polymer may be occasionally used for the substrate for the phase difference plate. Such a plastic including TAC has a lower heat resistance, as compared with glass. Therefore, when polyimide is obtained by applying a solution containing polyamic acid and a high boiling point solvent onto the substrate of the plastic and heating the substrate at a high temperature, use of the resultant polyimide as the liquid crystal alignment film has been difficult in some cases. Furthermore, TAC has a low solvent resistance. Therefore, a solvent that can be used is limited according to the method by which the film is prepared by applying an aligning agent to be the liquid crystal alignment film onto the substrate. For example, polyamic acid that has been used for forming polyimide so far has a low solubility in the solvent that can be applied to TAC, which has been a problem.